In the field of hydro forming, it is known to inject the incompressible liquid in a heated preform made of plastic material at a pressure which is adapted to urge the wall of the preform against the wall of the molding cavity such that the preform is deformed and acquires the shape of the molding cavity and of the container to be produced.
However, it is also known that this pressure level is not sufficient to completely shape the preform into the container, meaning that, with the pressure applied to the preform, the preform acquires a shape which is not exactly the shape of the molding cavity and that an extra deformation is needed to completely urge the wall of the preform against the wall of the molding cavity. It is in particular the case when embossed letters or logos must be reproduced on the external surface of the container wall, or when the wall has ridges. Shapes having locally a very small radius of curvature are very difficult to obtain.
To this end, after the injection of liquid at a first pressure, a second pressure greater than the first pressure, is applied to the preform during a short time in order to create a pressure peak inside the preform, the pressure peak being arranged to finalize the shaping of the preform into a the container.
The liquid is injected through the outlet of an injection device placed in fluidic communication with the preform and comprising at least a closing body movable between a closed position and an opened position controlling the injection of liquid inside the preform. A liquid tightness has to be established between the moving closing element and the fixed part of the injection device in order to avoid any leaking of the injected liquid, in particular to prevent liquid from flowing towards the actuation means controlling the movement of the closing body. Such a fluid tightness is achieved using sealing means arranged between the closing body and the fixed part of the injection device.
However, such sealing means have to be able to sustain the pressure of the liquid, in particular when the second pressure is applied to the preform, together with millions of cycles.
EP-1 762 539 discloses a filling device for injecting a liquid into an already formed container. In such a filling device, the liquid is injected at the atmospheric pressure since the container is not deformed by the liquid during the injection. Furthermore, in such a filling device, the outlet of the injection device is spaced from the inner volume of the container, meaning that the injection device is not in a fluid tight fluidic communication with the container during the liquid injection. Consequently, the sealing means provided in such a filling device does not have to sustain a high pressure during the injection. In EP-1 762 539, the sealing means are formed by a disc-shaped membrane arranged between a fixed part and a movable part, said membrane stretching and adopting a conical shape when the movable part moves to an opened position. Such a shape is not adapted to sustain a high liquid pressure and such sealing means would be rapidly deteriorated if they were to be used in an injection device arranged to form and fill a container from a preform.
WO-2008/078174 and U.S. Pat. No. 5,127,431 disclose sealing means arranged between a fixed part and a movable part and receiving liquid at a certain pressure. However, in these documents, the pressure applied on one side of the sealing means is compensated by the pressure of a fluid present on the other side of the sealing means. Such an arrangement is not satisfactory since it requires a control fluid to compensate the pressure of the injected liquid, which greatly complicates the structure of the injection device and the functioning thereof.
One of the aims of the invention is to overcome these drawbacks by proposing an injection device able to inject a liquid at a high pressure without damaging the sealing means and without requiring a compensating fluid to resist to the high pressure.